In recent years, there have been remarkable trends toward higher functions and higher performances of digital cameras and digital camcorders in which imaging devices such as CCDs and CMOSs are used. In particular, with the advance in semiconductor fabrication technology, high degrees of miniaturization are under way, and high degrees of integration are being attained, and an increase in the pixels of imaging devices from 1 million pixels to 10 million pixels is being made. On the other hand, in a production process of a multi-pixel imaging device, a higher pixel density induces more complicated production steps and a longer time for image quality checking, thus affecting the production yield and producibility.
Other than such multi-pixel based methods, higher-resolution images can also be obtained by a method of shifting the pixels of an imaging device, i.e., a so-called pixel shift technique. There are generally two kinds of pixel shift techniques. A first pixel shift technique consists in pixel shifts that involve shifts in positions within the image area, where, relative to a plurality of pixels which are periodically arrayed in an imaging surface of a solid-state imaging device, other pixels are placed so as to be shifted. A second pixel shift technique consists in dynamic pixel shifts, where at least one of an optical system and a solid-state imaging device of a two-dimensional square array is mechanically budged.
An example of the fundamental principles of the first pixel shift technique is described in Patent Document 1. In Patent Document 1, the first pixel shift technique is applied to a three-plate type color camera in which three imaging devices are used. This color camera adopts a configuration in which the pixels of an imaging device of green (G), which enjoys a high human luminous efficacy, are shifted by a 1/2 pitch along the horizontal direction in every other row, thus enhancing the resolution along the horizontal direction.
An example of shifting pixels not only along the horizontal direction but also along the vertical direction is shown in Patent Document 2. In a CCD imaging device of Patent Document 2, photodetecting portions corresponding to pixels are made into diamond shapes, and these are placed in a meandering shape. By disposing the pixels so as to be shifted by a 1/2 pitch of the pixels both along the horizontal direction and the vertical direction, the resolution along the horizontal and vertical directions is enhanced.
As for a dynamic pixel shift technique, an example of mechanically budging an optical system against an imaging device is described in Patent Document 3. In Patent Document 3, a translucent parallel-plate is placed between an imaging device and a lens. By swinging the parallel-plate relative to the optical axis, an optical image which is formed on the imaging device is budged, thus improving the resolution along the budging direction. An example of enhancing the resolution by budging the imaging device itself without moving the optical system is described in Patent Document 4. In this case, resolution is improved by using a piezoelectric element as the budging means, the amount of budging being a 1/2 pitch of the pixels.
Thus, in the conventional pixel shift techniques, through pixel placement or mechanical budging of the imaging device, pixels are shifted by a 1/2 pitch along the horizontal direction, or both horizontal and vertical directions, thus achieving an improved resolution. According to principles, if the opening ratio (pixel aperture ratio) of the photodetecting portions is 100%, the resolution will be increased twofold by shifting the pixels by a 1/2 pitch.
This will be described below with reference to FIG. 15 and equations. FIG. 15 is a graph showing a curve representing a relationship between the coordinate along a one-dimensional direction (X) of an image and a luminance value f(X), as well as a rectangular wave pulse waveform for sampling the luminance value. The horizontal axes X and t of the graph each represent distance from a certain reference point (e.g., the center of the imaging surface) along the horizontal direction. In the example of FIG. 15, with increasing X, the luminance value f(X) increases or decreases in sinusoidal waves.
One sampling pulse corresponds to one photodetecting portion. Although two sampling pulses are shown in FIG. 15, in actuality, a large number of sampling pulses exist. Herein, since the luminance value of an image which is formed on the imaging surface varies in sinusoidal waves along the X direction, the luminance value f(X) is expressed by eq. 1, using amplitude A, frequency ω, and phase 0. The sampling pulses are represented by a pulse height of 1, a frequency ωs, and a period (width) T.f(X)=A cos ωX  [eq. 1]
Since the received light amount of one photodetecting portion with an opening ratio of 100% equals an integrated amount for one period of sampling pulses, the received light amount P(n) of an nth pixel is expressed by eq. 2. By dividing the calculation result of eq. 2 with one period T and expressing it in terms of an average over time, eq. 3 is obtained. However, in eq. 3, B=2A/ωT. Furthermore, from the relationship of Tωs=2π, eq. 3 is expressed as eq. 4.P(n)=∫A cos ωtdt[t=−T/2+nT to T/2+nT]  [eq. 2]P(n)=B sin(ωT/2)cos(ωnT)  [eq. 3]P(n)=B sin(πω/ωs)cos(2πnω/ωs)  [eq. 4]
By varying n=0, 1, 2, . . . in eq. 4, the signal amount of an nth pixel is known. According to the sampling theorem, it is when ω/ωs≦1/2 that this signal amount is guaranteed. When ω/ωs>1/2, the signal difference from an adjoining pixel is small. When the value of ω/ωs approaches one, the signals of an nth pixel and an (n+1)th pixel are substantially in phase, so that there is hardly any difference therebetween.
When the pixels are shifted by a 1/2 pitch, n=n+1/2. By substituting this into eq. 4, eq. 5 is obtained, which expresses a received light amount after the pixel shift. That is, even when the value of ω/ωs approaches one, image information having a π shifted in its phase, which was not obtained from eq. 4, can be obtained. It will be appreciated that, because B sin(πω/ωs)=0 when ω/ωs=1, there is no resolving power in this case; however, resolution is possible until the value of ω/ωs is near one. Thus, it can be said that resolution can be increased nearly twofold by a pixel shift.P(n+1/2)=B sin(πω/ωs)cos(π(2n+1)ω/ωs)  [eq. 5]
In order to further confirm the above, simulation results of imaging a wedge-like resolution pattern while shifting the pixels by a 1/2 pitch, a 1/3 pitch, or a 1/4 pitch, where the photodetecting portion has an opening ratio of 100%, are shown in FIG. 16.
This simulation involves an imaging where the limit resolution along the horizontal direction in the case where no pixel shift is made is set to 200 lines. In FIG. 16, (A) shows an image where the pixels are shifted by a 1/2 pitch; (B) shows an image where the pixels are shifted by a 1/3 pitch, and (C) shows an image where the pixels are shifted by a 1/4 pitch. However, in the figure, in order to match the image size of (C), (A) and (B) are enlarged 2 times and 4/3 times, respectively, along the horizontal and vertical directions. From these results, it can be seen that the resolution is a little less than 400 lines in each case, which is an about twofold improvement from the limit resolution of 200 lines.
Thus, a pixel shift technique can improve resolution, but so long as the opening ratio is 100%, the resolution cannot be improved beyond twofold no matter how the pixel shift pitch is varied. Of course, the photosensitivity increases as the opening ratio increases. Therefore, in recent years, a microlens is provided on each pixel of the imaging device to increase the opening ratio to near 100%. However, from the standpoint of resolution improvement, not much improvement in image quality can be expected.
In such a situation, a technique for further improving the resolution is described in Patent Document 5. According to this technique, a pixel arrangement is adopted having a shift by a 1/2 pitch of the pixels along the horizontal direction and the vertical direction in the imaging surface. Furthermore, an opening having a minute opening ratio is provided at the center position between pixels, in order to obtain a photoelectric conversion signal also from such portions, thus further improving the resolution.
In this technique, the conventional pixel shift technique is adopted, and furthermore, minute openings are provided between pixels with some structural considerations. As a result, image information between pixels is obtained, whereby the resolution is commensurately improved. Other than resolution improvements, this technique also improves a so-called dynamic range because of being able to expand the luminance range for a subject to be imaged, with the minute openings and the photodetecting portions having usual openings.
Configurations having photodetecting portions with different opening ratios are also described in Patent Document 6 and Patent Document 7. According to these conventional techniques, images are reproduced by utilizing signals from the photodetecting portions having low opening ratios in a bright scene, and signals from the photodetecting portion having a high opening ratio in a dark scene, thus broadening the imaging conditions. In other words, the techniques disclosed in Patent Documents 6 and 7 are meant to improve the dynamic range in essence, and do not positively improve resolution.
Citation List
Patent Literature
                Patent Document 1: Japanese Laid-Open Patent Publication No. 58-137247        Patent Document 2: Japanese Laid-Open Patent Publication No. 60-187187        Patent Document 3: Japanese Laid-Open Patent Publication No. 63-284979        Patent Document 4: Japanese Laid-Open Patent Publication No. 64-69160        Patent Document 5: Japanese Laid-Open Patent Publication No. 2004-79747 (Japanese Patent No. 4125927)        Patent Document 6: Japanese Laid-Open Patent Publication No. 4-298175        Patent Document 7: Japanese Laid-Open Patent Publication No. 2006-174404        